Schema changes can take hours to apply if they are not done properly. If the developers tested the application of schema changes against a small database, they might not notice a potential problem. A small database can apply an inefficient schema update in a short period of time and will not raise any red flags. If you have a full-size test environment, you can apply the schema change and know approximately how long it will take to complete. It is important to understand how long this process takes, since the users of the application are locked out of parts of the system during the schema update.